The present invention relates generally to a scramjet combustor for a supersonic flight vehicle and more particularly to a scramjet combustor having improved combustor efficiency and to a method for its operation which optimizes combustor performance.
Although the theory of scramjet engines has been well known for many years, and although supersonic combustors have been tested in the laboratory, no scramjet engine is believed to have ever been successfully flown. Recent advances in technology, such as in high temperature materials, have made scramjet engines ready for implementation in the next generation of high speed aircraft. Such aircraft will be capable of flying at hypersonic speeds (i.e., speeds having Mach numbers greater than about 5.5). Hypersonic flight vehicles have been proposed which incorporate scramjet engines to achieve high Mach numbers. Once such a vehicle has achieved a sufficient speed by some other propulsive means (which may include a turbojet engine), a scramjet engine takes over to propel the aircraft to high Mach numbers (typically between Mach 6 and Mach 20). Such high Mach numbers cannot be achieved by any other known type of air-breathing engine.
A typical scramjet engine includes a combustor having a chamber, wherein a fuel-air mixture moving at supersonic speed is burned, and having at least one fuel injector which directs supersonically-moving fuel (such as pressurized hydrogen) into the chamber. The engine also includes an air inlet, which delivers compressed supersonically-moving air to the combustor chamber, and includes an exhaust nozzle, which channels the burning gases out of the combustor chamber to produce engine thrust. The fuel injectors are the nozzle parts of the combustor to which fuel is delivered by a fuel system which may include tanks, pumps, and conduits.
An important component of the scramjet engine is its combustor. The basic scramjet combustor of the literature includes a longitudinally-extending rectangular duct which defines the combustor chamber. The combustor's fuel injectors inject fuel into the combustor chamber through openings in the duct's two opposing larger walls. The longitudinally-moving air, from the engine inlet, and the typically longitudinally-or-transversely-injected fuel, from the fuel injectors, mix in the combustor chamber. In the case of hydrogen fuel, the fuel-air mixture in the combustor chamber will have a high enough temperature and pressure to auto-ignite.
The efficiency of burning within the combustor depends in part on how well the air and fuel mix. To promote mixing, a scramjet combustor disclosed in the literature has included angled fuel injection which means that the injected fuel is not parallel or perpendicular to the longitudinally-moving air. Another approach disclosed in the literature to promote better fuel-air mixing and burning stability has included an aft-facing step in one of the larger walls with (or without) the addition of angled fuel injection at the step location. An additional scramjet combustor, disclosed in the literature without elaboration, has included an aft-facing step in each larger wall, with the steps being a longitudinal distance apart.
The efficiency of burning within the combustor also depends in part on how much the air in the fuel-air mixture is compressed (increase in static pressure). The more the air can be compressed (within the limit of temperature at which the air dissociates) before the fuel-air mixture is burned, the more efficient and powerful the scramjet engine will be. The air compression disclosed in the literature has been accomplished by the rectangular, funnel-like inlet portion of the engine which leads from the engine inlet's entrance, where the engine inlet opening is largest, to the engine inlet's throat, where the engine inlet opening is smallest. The inlet of a scramjet engine may have fixed or variable geometry. Variable geometry means that the engine throat area may be changed, and there is an optimal throat area for any given set of flight conditions, as is known to those skilled in the art. A scramjet with a variable geometry engine inlet can operate, and operate more efficiently, over a greater range of flight conditions than can a scramjet with a fixed geometry engine inlet. However, if the inlet throat area is made too small, air boundary layer instability or choking (reducing the airflow to sonic speed) in the inlet throat will exist, causing inlet unstart. This means that if the inlet compresses the air too much (because of a too small throat area) at lower Mach numbers, the scramjet engine cannot be started.
Although scramjet combustor designs have been proposed which increase combustor efficiency, none are known which optimize combustor efficiency by optimizing the parameters of step separation, fuel injection angle, and wall separation for design (cruise) flight conditions. Also, no scramjet combustors are known which are self-adaptive for such parameters. This means none are known to have variable geometry for such parameters enabling their configurations to be changed during supersonic flight to maintain optimized combustor efficiency during changes in flight conditions. Changes in flight conditions include, for example, changes in the combustor inlet Mach number during the acceleration-to-cruise phase of a flight.